fix: astype Decimal to Int64 conversion. (#957)

* fix: astype Decimal to Int64 conversion.

* update format

Author: Genesis929

bigframes/core/compile/ibis_types.py

@@ -144,10 +144,12 @@ def cast_ibis_value(
         ),
         ibis_dtypes.Decimal(precision=38, scale=9): (
             ibis_dtypes.float64,
+            ibis_dtypes.int64,
             ibis_dtypes.Decimal(precision=76, scale=38),
         ),
         ibis_dtypes.Decimal(precision=76, scale=38): (
             ibis_dtypes.float64,
+            ibis_dtypes.int64,
             ibis_dtypes.Decimal(precision=38, scale=9),
         ),
         ibis_dtypes.time: (

tests/system/small/test_series.py

@@ -3080,6 +3080,16 @@ def test_astype(scalars_df_index, scalars_pandas_df_index, column, to_type):
     pd.testing.assert_series_equal(bf_result, pd_result)
 
 
+@skip_legacy_pandas
+def test_astype_numeric_to_int(scalars_df_index, scalars_pandas_df_index):
+    column = "numeric_col"
+    to_type = "Int64"
+    bf_result = scalars_df_index[column].astype(to_type).to_pandas()
+    # Round to the nearest whole number to avoid TypeError
+    pd_result = scalars_pandas_df_index[column].round(0).astype(to_type)
+    pd.testing.assert_series_equal(bf_result, pd_result)
+
+
 @pytest.mark.parametrize(
     ("column", "to_type"),
     [